Corrosion under insulation (CUI) is a condition in which an insulated structure such as a metal pipe suffers corrosion on the metal surface beneath the insulation. As the corrosion cannot be easily observed due to the insulation covering, which typically surrounds the entire structure, CUI is challenging to detect. The typical causes of CUI are moisture buildup that infiltrates into the insulation material. Water can accumulate in the annular space between the insulation and the metal surface, causing surface corrosion. Sources of water that can induce corrosion include rain, water leaks, and condensation, cooling water tower drift, deluge systems and steam tracing leaks. While corrosion usually begins locally, it can progress at high rates if there are repetitive thermal cycles or contaminants in the water medium such as chloride or acid.
Studies indicate that 40 to 60% of pipe maintenance costs are caused by CUI. When CUI is undetected, the results of can lead to the shutdown of a process unit or an entire facility, and can lead to catastrophic incidents. Since it is a hidden corrosion mechanism, the damage remains unnoticed until insulation is removed or advanced NDT (non-destructive testing) techniques, such as infrared thermography, are used to ascertain the metal condition beneath the insulation. Removal of insulation can be a time-consuming and costly process, while the accuracy NDT techniques can be insufficient due to the large number of variables (e.g., geometrical, environmental, material-related), that cause false positives (incorrect detection of corrosion) and false negatives (incorrect non-detection of corrosion) in the detection process.
What is therefore needed is improvement in the NDT detection process that will enable external and confounding variables to be more properly accounted for so as to improve the accuracy of CUI detection.